1. Field of the Invention
The invention relates to a process for the treatment of a gas stream, in which the gas stream is passed over a catalytic adsorber module to oxidize entrained impurities. It further relates to a gas treatment system suitable for carrying out the process.
2. Summary of the Invention
It is accordingly an object of the invention to provide a method for treating a flow of gas and a gas treatment system that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type.
In the operation of a nuclear plant, in particular a nuclear power station, a customary design objective is the best possible avoidance of corrosion damage on important components, especially in the primary region of the respective plant, for example on graphite internals, fuel assemblies or other components in the pressure vessel of the reactor. This is because substantial avoidance of corrosion damage on these components should increase the life or operating time and keep down the maintenance and repair requirement associated with the alleviation of corrosion damage in the primary region of the nuclear plant, which is sometimes considerable. For this reason, the use of helium may be specified as a working or cooling medium in a nuclear plant, in particular in the primary circuit of a high-temperature reactor. This is because helium is chemically inert, so that, for example, corrosion phenomena caused by the cooling gas on the components specified does not have to be reckoned with when helium is used as the cooling gas for these components.
However, impurities such as carbon monoxide (CO), molecular hydrogen (H2), methane (CH4), molecular oxygen (O2), tritium, water (H2O), carbon dioxide (CO2) and/or dust particles can get into the helium used as the primary coolant or cooling gas during operation of the nuclear plant, in particular during operation of a high-temperature reactor. The impurities can in turn lead to undesirable corrosion phenomena on the components specified. To keep these effects small and, in particular, below prescribed limits which are considered to be still permissible, limitation of the concentration of such impurities in the cooling gas stream by use of a gas purification plant or a gas treatment system may be specified.
During operation of such a gas purification system, a substream of from about 50 kg/h to 300 kg/h is usually taken from the helium cooling circuit and first passed through a dust filter. The gas stream to be purified is subsequently heated to a temperature of about 250° C. and fed to a catalytic adsorber module. The catalytic adsorber module serves first to catalyze the transformation processes provided and second as a type of buffer for the temporary storage of oxygen required in these processes. In the catalytic adsorber module, which usually contains a Cu—CuO mixture as catalytically active adsorber component, oxidation of the hydrogen and carbon monoxide entrained as impurities in the gas stream to be purified to water (H2O) and carbon dioxide (CO2) occurs under the specified, suitably selected operating temperature. The oxygen required for this is taken from the CuO of the catalytically active adsorber material, so that a continuous increase in the proportion of Cu at the expense of the CuO occurs as a result of the reaction. The gas stream to be purified, which has now been freed of molecular hydrogen and carbon monoxide, is then usually cooled, with the entrained water and carbon dioxide being separated out in molecular sieves. A low-temperature adsorption in which predominantly methane, molecular oxygen and association products are removed by adsorption from the gas stream to be purified is then usually carried out. After the removal of the impurities is complete, the now purified gas stream is returned to the helium cooling circuit.
However, such a gas treatment system is comparatively complicated, especially in respect of the number and installation of the components required. In addition, the use of the catalytic adsorber module of the type mentioned results in that regeneration by treatment with oxygen is necessary after the CuO in the catalytic adsorber module has been “consumed”, i.e. after virtually all the CuO has been converted into Cu, so that the respective module is not available for purification of the gas stream during this time. For this reason, two or more similar sub-lines are usually connected in parallel in such a gas treatment system, which increases the outlay for apparatus even further.
It is therefore an object of the invention to provide a process for the treatment of a gas stream of the above-mentioned type by which reliable purification of the gas stream is made possible using comparatively simple apparatus. Furthermore, a gas treatment system, which is particularly suitable for carrying out the process is to be provided.
According to the invention, this object is achieved in respect of the process by passing the gas stream over a first catalytic adsorber module in a first purification stage for the oxidation of entrained impurities and mixing molecular or atomic oxygen into the gas stream, with the gas stream which has been admixed with the mixed-in oxygen being passed over an oxidation catalyst in a second purification stage and the gas stream leaving the oxidation catalyst being passed over a second catalytic adsorber module in a third purification stage for the reduction of excess oxygen.
The invention starts out from the idea that the outlay in terms of apparatus and operation for reliable purification of the gas stream using catalytic adsorber modules can be kept particularly low by, in particular, keeping the total number of components required small. The concept of gas treatment should therefore be directed at substantial avoidance of redundancies in the components used. To accordingly keep the number of sub-lines having the same effect which are connected in parallel particularly small or to be able to configure the gas treatment system as a single line in terms of the gas flow, the concept for the treatment of the gas stream should be directed at continuous operation of the respective catalytic adsorber module. This can be achieved by the use of two catalytic adsorber modules connected in series in the direction of gas flow, of which one is used in a conventional manner for the oxidation of the entrained impurities in the treatment of the gas stream and is reduced as a result, while the other catalytic adsorber module is used for the reduction of oxygen and is oxidized as a result.
When one of the adsorber modules is completely “exhausted”, i.e. the respective constituent is completely oxidized or reduced, in such a configuration, continued operation of the gas treatment system is made possible by simple reversal of the gas flow through the catalytic adsorber modules. To make the combined use of the catalytic adsorber modules for oxidation and reduction, respectively, possible, the gas stream is subjected to a further purification stage, which is required in any case, in an oxidation catalyst between the catalytic adsorber modules. The additional oxygen required for this purpose is mixed into the gas stream at a suitable point upstream of the oxidation catalyst, with excess oxygen being available in the second, downstream catalytic adsorber module for oxidation and thus regeneration of the latter.
A Cu—CuO mixture is advantageously used as the catalytic adsorber material both in the first catalytic adsorber module and in the second catalytic adsorber module. In the first catalytic adsorber module viewed in the direction of gas flow, which is provided for oxidation of impurities entrained in the gas stream, the CuO in the adsorber material is converted into Cu with liberation of the oxygen required for oxidation. In contrast, in the second catalytic adsorber module viewed in the direction of gas flow, in which the excess oxygen now present in the gas stream is removed by adsorption, the Cu in the catalytic adsorber material is converted into CuO. As the time of operation of the treatment of the gas stream increases, the proportion of Cu in the first, upstream catalytic adsorber module increases and the proportion of CuO in the catalytic adsorber material present there decreases, while, conversely, the proportion of Cu in the second, downstream catalytic adsorber module decreases and the proportion of CuO in the catalytic adsorber material present there increases. If one of the catalytic adsorber modules is found to be “exhausted”, i.e. the respective catalytic adsorber material present has been completely converted into Cu or CuO, the gas flow through the catalytic adsorber modules can be switched over, so that the CuO-enriched catalytic adsorber module is now used as first catalytic adsorber module for the oxidation of impurities entrained in the gas stream and the Cu-enriched catalytic adsorber module is used as second catalytic adsorber module for the reduction of excess oxygen.
Oxygen is advantageously mixed in in such an amount that sufficient excess oxygen is always present in the second, downstream catalytic adsorber module to oxidize the catalytically active adsorber material present there. For this purpose, it is advantageous to determine an index for the proportion of entrained impurities in the gas stream before the gas stream enters the first catalytic adsorber module, by which the amount of oxygen to be mixed into the gas stream settles. To ensure efficient utilization of both the first catalytic adsorber module and also the second catalytic adsorber module, the amount of oxygen mixed in is advantageously set so that there is a deficiency of oxygen based on the total impurities entrained in the gas stream and thus at least part of the oxidation of the impurities occurs in the first catalytic adsorber module and so that there is an oxygen excess based on the reaction of further impurities intended to occur in the oxidation catalyst, so that excess oxygen is available in the second catalytic adsorber module for regeneration of the catalytic adsorber material present there.
The oxidation catalyst is preferably used for the treatment of impurities such as methane or tritium. To ensure a particularly high conversion and thus particularly careful removal of such impurities from the gas stream, the temperature of the gas stream is advantageously set to from about 400° C. to 450° C. before it enters the oxidation catalyst, so that, when sufficient oxygen has been made available, particularly substantial conversion of the impurities mentioned into water and carbon dioxide can occur. A particularly resource-conserving and thus economical mode of operation can be achieved in a particularly advantageous embodiment by the gas stream being preheated by recuperative heat exchange with the gas stream leaving the oxidation catalyst before the first gas stream enters the oxidation catalyst. The heat contained in the gas stream leaving the oxidation catalyst is at least partly utilized for preheating the gas stream entering the oxidation catalyst, so that supplementary heating, for example electric supplementary heating, may still be necessary for setting the final desired entry temperature in the gas stream.
Particularly when using a Cu—CuO mixture as the catalytically active adsorber material in the first catalytic adsorber module, this is preferably used for the oxidation of hydrogen and carbon monoxide entrained in the gas stream. To ensure a particularly favorable reaction rate and a particularly favorable degree of reaction in the oxidation of these to water and carbon dioxide with targeted utilization of the catalytic properties of Cu, a temperature of about 250° C. is advantageously set for the gas stream before it enters the first catalytic adsorber module. Here too, a particularly resource-conserving and thus economical mode of operation can be achieved in a further advantageous embodiment by the gas stream being preheated by recuperative heat exchange with the gas stream leaving the second catalytic adsorber module before the first gas stream enters the first catalytic adsorber module. Thus, in this advantageous embodiment, the heat content of the total gas stream leaving the second catalytic adsorber module and thus the gas purification system is utilized for the partial preheating of the gas stream flowing into the gas purification system.
In a particularly advantageous embodiment, the process is used in the operation of a nuclear power plant for the treatment of a substream of a helium cooling gas stream. Here, the substream of the helium cooling gas stream is preferably freed of entrained impurities such as carbon monoxide, molecular hydrogen, methane, molecular oxygen, tritium, water and/or carbon dioxide. The conversion of molecular hydrogen and carbon monoxide into water and carbon dioxide is preferably effected in the first catalytic adsorber element. When a sufficient amount of oxygen is mixed in in good time, methane and/or tritium are then likewise converted into carbon dioxide and/or water in the oxidation catalyst. The excess oxygen, which then still remains in the gas stream is subsequently used for enrichment of the second catalytic adsorber module and thus removed from the gas stream again. Removal of the water and carbon dioxide still present in the gas stream can subsequently be effected in a conventional way, and can, if appropriate, be supplemented by removal of dust particles or noble gas activities. The helium gas substream, which has been purified in this way is subsequently returned to the actual helium cooling circuit.
In this application in particular, the process can be utilized in a particularly advantageous way to make continuous treatment of a gas stream possible while using comparatively few components. Since the first catalytic adsorber module viewed in the flow direction of the gas stream is reduced in the treatment of the gas stream but the second catalytic adsorber module viewed in the flow direction of the gas stream is oxidized, CuO is continuously converted into Cu in the first catalytic adsorber module and Cu is continuously converted into CuO in the second catalytic adsorber module.
As soon as it has been established that one of the catalytic adsorber modules is completely “exhausted”, i.e. the Cu or the CuO has been converted completely into the other component of the mixture, the order in which the catalytic adsorber modules are connected in the flow path of the gas stream can be switched around. After switching has been carried out, the second catalytic adsorber module which has hitherto been utilized for removal of oxygen from the gas stream is thus utilized further as newly connected first catalytic adsorber module, with the oxygen incorporated in this adsorber module now being given off again to the gas stream in the treatment of the relevant impurities in the gas stream. The first catalytic adsorber module which has hitherto been used for the oxidation of hydrogen or carbon monoxide in the gas stream is, after switching over, utilized as newly connected second catalytic adsorber module, with the CuO present in the catalytically active adsorber material being regenerated by uptake of the excess oxygen from the gas stream.
To achieve timely and particularly appropriate implementation of the switch-over, an index for the proportion of possibly entrained oxygen is advantageously determined for the gas stream leaving the second catalytic adsorber module. After a prescribed limit for this index has been exceeded, it is concluded that the Cu in the second catalytic adsorber module has been completely reacted, so that the positions of the first and second catalytic adsorber elements in the flow path of the gas stream are exchanged.
With regard to the gas treatment system, the object indicated is achieved by at least two catalytic adsorber modules, which are connected in series in the direction of a gas stream and between which an oxidation catalyst is located.
To aid the intended conversion of the respective impurities in the oxidation catalyst to a particular degree, the oxidation catalyst is advantageously preceded in the gas flow direction by a feed unit for molecular or atomic oxygen (test, see above). In a particularly advantageous embodiment, the introduction or mixing-in of the oxygen into the gas stream is carried out in the amount required and thus as a function of the impurities entrained in the gas stream. To make this possible, a control parameter transducer assigned to the feed unit is advantageously connected on the inlet side to a sensor for the proportion of entrained impurities in the gas stream located upstream of the first, upstream catalytic adsorber module.
The gas treatment system is advantageously equipped for use in the treatment of a substream from a helium primary cooling circuit of a nuclear plant. For the removal of typical impurities such as molecular hydrogen or carbon monoxide from a helium gas stream in particular, first the catalytic properties and second the suitability for storage of oxygen of a Cu—CuO mixture are particularly advantageous. The catalytic adsorber modules of the gas treatment system therefore advantageously each contain a Cu—CuO mixture as catalytic adsorber material.
To make it possible to set particularly advantageous and appropriate operating parameters, in particular a suitable operating temperature in the oxidation catalyst, the oxidation catalyst is advantageously preceded in the gas flow direction by an intermediate heating system. This can be operated in a particularly resource-conserving and thus economical manner when it is configured for the recovery of heat from the gas stream leaving the oxidation catalyst. To achieve this, the intermediate heating system advantageously contains a recuperative heat exchanger, which is connected on the primary side into an outflow line for the gas stream from the oxidation catalyst and on the secondary side into an inflow line for the gas stream to the oxidation catalyst.
To make it possible for a desired entry temperature of the gas stream into the oxidation catalyst to be set appropriately in a flexible mode of operation, the recuperative heat exchanger is supplemented in a further advantageous embodiment by a heating element, advantageously an electric heater. In an analogous manner, the gas treatment system is also configured for the setting of a particularly favorable operating temperature in the first, upstream catalytic adsorber module. For this purpose, it is advantageously preceded by a heating system. The heating system, too, can be operated in a particular resource-conserving and thus economical manner by being advantageously configured for the recovery of heat from the gas stream leaving the gas treatment system. To achieve this, the heating system contains, in a further advantageous embodiment, a recuperative heat exchanger, which is connected on the primary side into an outflow line for the gas stream from the second catalytic adsorber module and on the secondary side into an inflow line for the gas stream into the first catalytic adsorber module.
In a particularly advantageous embodiment, the gas treatment system is configured for a continuous mode of operation in which the first, upstream catalytic adsorber module is reduced during operation and the second, downstream catalytic adsorber module is oxidized. To make continuous operation possible even after complete reaction of the active materials in these reactions, the gas treatment system is advantageously configured for switching of the catalytic adsorber modules in respect of their connection in the flow path of the gas stream when necessary. For this purpose, the catalytic adsorber modules are advantageously provided with a joint switching system for directing the flow of the gas stream.
A particularly compact construction of the gas treatment system can be achieved in a particularly advantageous embodiment by its components, i.e. in particular the catalytic adsorber modules and the oxidation catalyst but also, if applicable, the heating systems with their heat exchangers and/or the feed unit for oxygen, being disposed in an integrated configuration in a common pressure vessel. Here, all components mentioned can be surrounded by a common pressurized enclosure which ensures maintenance of the pressure of the total system.
The individual components located in the high-pressure enclosure can, as a result of the decoupling of the maintenance of the pressure from the structural configuration of the individual components, have comparatively thin walls and be designed for low mechanical stresses. This first allows a material-saving and thus economical construction and second, owing to the then comparatively small thermal masses, comparatively rapid heating and cooling of individual components and rapid and flexible matching of the reaction temperatures required for the gas stream to be purified. In particular, a thin-walled configuration of the active components makes it possible for a comparatively high temperature to be set quickly and reliably in the respective reaction zones, so that a comparatively high conversion in the individual reactions can be achieved even in the short term.
In addition, the integration of the recuperative heat exchanger into the common pressurized enclosure also allows effective cooling of the outflowing gas stream before it reaches downstream purification components, for example molecular sieves, with effective heating of the inflowing gas stream being ensured at the same time. The recuperative heat exchanger located upstream of the oxidation catalyst likewise makes it possible to achieve effective cooling of the gas stream leaving the oxidation catalyst, so that overheating of the second catalytic adsorber module located downstream of this can be reliably avoided.
The gas treatment system is advantageously connected to the helium cooling gas circuit of a nuclear plant.
The advantages achieved by the invention are, in particular, that the connection of an oxidation catalyst in series between a first catalytic adsorber module and a second catalytic adsorber module makes targeted treatment of various impurities in the respective gas stream possible, with the first, upstream catalytic adsorber module being able to be used for oxidation purposes in the treatment of the gas stream and its oxygen-carrying component thus being increasingly consumed but the second, downstream catalytic adsorber module being at the same time able to be used in converse operation to remove excess oxygen from the gas stream, thus regenerating its oxygen-carrying component.
The normal operation of a catalytic adsorber module and the regeneration of a catalytic adsorber module are thus effected at the same time and thus in one process operation. Even after “consumption” of the oxygen-carrying component has occurred in the first, upstream catalytic adsorber module, the gas treatment system can be utilized further without an appreciable interruption to operation after the positions of the catalytic adsorber modules in the flow path of the gas stream have simply been switched over so that the previously regenerated adsorber module is now used as first, upstream catalytic adsorber module for the oxidation of impurities in the gas stream and the “exhausted” catalytic adsorber module is now regenerated at the same time. As a result of the continuous operation of the gas treatment system, in which interruptions to operation for specific regeneration of individual adsorber modules are no longer necessary, made possible in this way, redundancies or multi-stream configurations of such systems can be dispensed with or at least reduced. In addition, the integration of the active components into a common pressurized enclosure achieves a particularly compact and thus space-saving construction in which particularly simple and rapid process operation is additionally made possible as a result of the decoupling of maintenance of the pressure from the thermally stressed structural components.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for treating a flow of gas and a gas treatment system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.